Sigurdsson made his second debut for Swansea on Sunday, scoring twice in a 4-0 friendly win over Plymouth Argyle having rejoined from Tottenham earlier in the week as Ben Davies and Michel Vorm headed in the opposite direction.
The Iceland international proved a success during a loan spell at the Liberty Stadium in 2012, but Monk feels Sigurdsson is now even better.
"He's come in and contributed straight away. He adds quality to our squad, which is what we want," Monk told Swansea's official website.
"He's a top player – actually a better player than when he was first here.
"It helps that he already knows how this club works. He knows me well, as well as a few of the boys, so in terms of settling in, that will help him."
Monk took plenty of positives from his side's routine victory over Plymouth, which also featured goals for Rory Donnelly and Jordi Amat.
"We're getting there. We controlled the game from start to finish and ended up with a nice result," he added.
"It was a good workout. Plymouth worked us hard, particularly in the first half."